1. Field of the Invention
The present invention relates in general to integrated circuits and in particular to a temperature control system for an integrated circuit.
2. Description of Related Art
In many applications it is important for transistors forming logic circuit to switch with constant, predictable switching speeds. However, the switching speed of a transistor, particularly a metal oxide semiconductor (MOS) transistor, is highly temperature sensitive; as a transistor warms up, it reduces the speed with which it turns on an off. A transistor""s temperature is influenced, for example, by its ambient temperature, the size, number and proximity of heat sources and sinks in its immediate vicinity, and by the heat transfer efficiency of its surrounding media. Since a transistor itself is a variable heat source, generating substantial heat when it is on and very little heat when it is off, the manner at which the transistor is operated can also influence its temperature. As the duty cycle of a transistor increases, it tends to generate more heat, warm itself up, and therefore slow down. This effect is tempered to some extent in complementary metal oxide semiconductor (CMOS) integrated circuits where n-channel (nMOS) and p-channel (pMOS) transistors forming logic gates are paired and arranged such that the nMOS and pMOS transistors of each pair have opposite switching states. When one transistor of a pair is on, the other transistor of the pair is off. Thus the amount of heat the pair generates tends to be independent of the switching states of the transistors, provided the transistors are well matched.
Nonetheless, the temperature of a CMOS circuit can fluctuate not only due to variations in its ambient temperature but also due to changes in its frequency of operation. When a CMOS transistor pair changes state, the pMOS transistor turns on (or off) at the same time the nMOS transistor turns on (or off). But the two transistors do not turn on or off instantly; there is a period of time during a state change when both transistors are in their active regions, between fully off and fully on. During that time both transistors generate heat, and the total amount of heat they collectively generate per unit time is greater during state changes than between them. CMOS logic gates change state when their input signals change state. Thus as the frequency of a CMOS logic circuit input signal increases, the rate at which the logic gates forming that circuit change state also increases, and so too does the heat the logic gates generate. That is why CMOS circuits get hotter when they operate at higher frequencies. Therefore, in applications requiring a high degree of switching speed stability, it is helpful to control the temperature of an integrated circuit (IC), particularly when a circuit""s inputs change state at varying frequencies.
FIG. 1 illustrates a well-known feedback system for controlling the temperature of a CMOS or other kind of IC 10 so as to stabilize the switching speed of its transistors. A sensor 12 monitors the temperature of IC 10 and generates an indicating signal (IND) having a parameter indicative of the IC""s temperature. Sensor 12 may be, for example a thermistor in contact with IC 10. A control circuit 14 compares the IND signal to a reference signal (REF) and supplies an output control signal (CONT) to an external heater 16 near IC 10. When the magnitude of the IND signal exceeds the magnitude of the REF signal, indicating for example that IC 10 is hotter than desired, control circuit 14 signals heater 16 to reduce the rate at which it generates heat, thereby allowing IC 10 to cool down. Conversely when the magnitude of the IND signal falls below the magnitude of the REF signal, indicating that IC 10 is cooler than desired, control circuit 14 signals heater 16 to increase the rate at which it generates heat, thereby warming IC 10. The magnitude of REF therefore controls the temperature of IC 10.
One limitation on the ability of the feedback system of FIG. 1 to closely control IC temperature is that it takes time for a change in temperature of IC 10 to be detected by sensor 12, and additional time for a change in heat generated by the external heater 16 to influence the temperature of IC 10. This feedback delay limits the accuracy with which the feedback system can control the temperature of IC 10 when the rate at IC 10 generates internal heat changes rapidly, as when there is an abrupt change in input signal frequency. The temperature of circuits in integrated circuit testers and other applications that change abruptly from inactive to full speed operation can undergo a rapid swing before the feedback system of FIG. 1 has time to compensate for the changed operating conditions.
FIG. 2 illustrates an improved prior art temperature control system for an IC 18 that is topologically similar to that of FIG. 1 except that a sensor 20, control circuit 22 and heater 24 are implemented within IC 18 itself, with sensor 20 and heater 24 positioned as close as possible to the logic circuits 26 being temperature controlled. Sensor 20 monitors the temperature of logic circuit 26 and generates an indicating signal (IND) having a parameter indicative of the IC""s temperature. For example sensor 20 may be a diode and the IND signal may be the diode""s threshold voltage, a parameter that is highly sensitive to temperature. Or, as another example, sensor 12 may be a ring oscillator formed by gates implemented on IC 18 having a temperature sensitive frequency. Heater 24 can be implemented as a set of transistors that are turned on and off by the control signals CONT. This design reduces the feedback lag between temperature changes in logic circuit 26 and compensating changes the heat it receives from heater 24. However while sensor 20 may directly or indirectly sense the temperature of a particular part of IC 18, the sensed temperature may not be representative of the temperature or switching speed of all transistors on IC 18. The transistors forming logic circuit 26 are distributed in space and some of those transistors may be nearer sensor 20 or heater 24 than others. Also at various times the IC input signals may increase the frequency of operation of some portions of logic circuit 26 while decreasing the frequency of others. Thus the rate of internal heat generation within logic circuit 26 can vary from area-to-area of IC 18. When sensor 20 senses one area of IC 18 growing colder, controller 22 tells heater 24 to generate more heat, even though some parts of logic circuit 26 may already be growing warmer.
FIG. 3 illustrates a prior art temperature compensation system wherein the logic of an IC 30 is organized into a set of logic blocks 32, with each logic block 32 being provided with its own sensor 34, controller 36 and heater 38. Since this system localizes sensing and heating, the various logic blocks 32 are better controlled than in the centralized systems of FIGS. 1 and 2. While the system of FIG. 3 can be generally quite effective in controlling temperature of an IC subject to a slowly changing thermal environment, its ability to control the temperature of the transistors forming a given logic block 32 is still limited to some extent by lags in the feedback loop provided by sensor 34, controller 36 and heater 38. This feedback system is particularly vulnerable when a sudden change in the input signal frequency of a logic block 32 results in a sudden change in the amount of heat the logic block generates. Such an event can cause the transistor switching speed to briefly go out of its acceptable range before the feedback system has had time to detect the temperature change and to adjust the heat flow into the logic block.
What is needed is an improved temperature compensation system that quickly responds to changes in input signal frequency.
In accordance with one aspect of the invention, a temperature control system for an integrated circuit (IC) having one or more digital logic modules provides a separate heater circuit near each logic module for heating the logic module and a separate predictive control circuit for controlling each heater circuit. Each predictive control circuit monitors input signals supplied to its corresponding logic module. When any input signal changes state, the predictive control circuit signal the logic module""s heater to temporarily reduce its heat output. A change in state of an input signal to a logic module will cause the logic module to temporarily increase the rate at which it generates heat, but the temporary reduction in heat generation rate of the heater circuit offsets the temporary increase in heat production of the logic module, thereby reducing logic module temperature fluctuation.
In accordance with another aspect of the invention, a feedback system senses the temperature of each logic module and further adjusts the rate at which the module""s heater produces heat so as to maintain the logic module at a substantially constant temperature.
The temperature control system of the present invention is particularly suitable for controlling temperature of circuits in integrated circuit testers and other applications that change abruptly from being relatively inactive to full speed operation. Unlike systems employing only temperature sensing feedback, the system of the present invention predicts when a circuit is going to change its heat output and makes appropriate heater adjustments when the circuit changes its heat output rather than waiting until after the circuit temperature has already begun to change.
It is accordingly an object of the invention to provide a temperature control system for an integrated circuit which holds digital logic modules therein at a substantially constant temperature despite abrupt changes in the switching frequency of the module""s input circuits.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the. invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.